[2008] Arithmetic operation resulted in an overflow

[perito]

When the loop is
For x = 3 to 10
the codes work fine

when i change it to 100, the code gives an eroor
"Arithmetic operation resulted in an overflow."
on the line "If (Result + PreResult) Mod 2 = 0 Then y = y + Result + PreResult"

WHY????

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim x As Integer, Result As Long, PreResult As Long, oldresult As Long
        Dim y As Long
        Result = 2
        PreResult = 1
        y = 3
        For x = 3 To 100
            If (Result + PreResult) Mod 2 = 0 Then y = y + Result + PreResult
            oldresult = PreResult
            PreResult = Result
            Result = Result + oldresult
        Next
        TextBox1.Text = y
    End Sub

[Atheist]

WHY????

Because 'Result' will hold 7540113804746346429, and 'PreResult' will hold 4660046610375530309. And you're trying to add them together. Thats a pretty big number.

Besides, the logic in that loop is pretty..odd, what are you trying to accomplish?

[Shaggy Hiker]

Put a breakpoint on the line Result = result + oldresult, then step through the code watching the size of result balloon. That math is not quite a binary expansion, but it is close, because it is similar to n += 2n, which, for any n>1 is a binary sequence, which means that on the 100th iteration, n holds 2^99, but a long integer can only hold about 2^63, so it overflows long before you reach the 100th iteration.

[Shaggy Hiker]

I knew I'd be too slow.

[perito]

The question is this

Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:

1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...

Find the sum of all the even-valued terms in the sequence which do not exceed one million.


i cant think of a better way to do this...
please give me new ideas

[Shaggy Hiker]

But you don't need to go past 1,000,000, and you are currently going FAR beyond that.

Just add a condition into your loop to see if the value has gone over one million, and exit for if it has. Alternatively, rather than using a For Next loop, since you don't really know how many times you are going to loop, use a Do...Loop. Have x just be a counter in there that gets incremented each time through the loop, and loop until the value exceeds one million. That's a better solution than using the Exit For, though they will do about the same thing.

[perito]

hmmm....
i need to loop a million times right? because in the example there are 10 numbers, the sum of the even is 44

to do that, we loop 10 times, check if the number is even, add it.
I didnt understand what u mean...
Do Until y>= 1000000 will dont work...

please can u explain more

[Shaggy Hiker]

I read your last post to suggest mean that you are looking for all numbers less than one million in the sequence, rather than looking for the first million numbers in the sequence.

Do While Y<= 1000000

would get you pretty close to the result, however it will end with the first value of y that is greater than 1000000, and it looks like you want the result just before that one. Therefore, it seems like a safer loop would be:

DO

Loop While (y + result + oldresult <= 1000000)

Since it appears that as soon as that becomes false, then the next value of y will be greater than 1,000,000, so the current value of y will be the largest one that is less than 1,000,000.

[perito]

the answer is still wrong :S
i even tried
Loop While (y + Result + oldresult <= 1000000)
y = y - oldresult - PreResult
and its still wrong.

Im wondering, the question might be different
Find the sum of all the even-valued terms in the sequence which do not exceed one million.

they might mean that the sequence doesnt exceed one million, not the sum. that will result in a different code.
ideas?

[Shaggy Hiker]

Oh, I hadn't seen that. Now I get it.

In y, you are creating the sum of all the values. In Result you are getting the next item in the sequence. At the end of the loop:

Result: Holds the next item in the sequence (item N).
PreResult: Holds the last item in the sequence (item N-1).
OldResult: Holds the second to last item (item N-2).

So your sequence is correct, but what about your If statement?

Why are you checking Result + PreResult Mod 2? Result is the next item in the sequence, so shouldn't the if be:

If Result Mod 2 = 0 Then Y += Result

Then the loop would be:

Do

Loop while Result <= 1000000

[perito]

no the final result is (Result + PreResult)
make the loop until 10 and u will see that
Result + PreResult = 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
Result alone is meaningless
i made the changes and checked anyway and the answer is still wrong...

[Shaggy Hiker]

It works as it does because you are "looking ahead" at the next result, and adding it in if it belongs, whereas I was suggesting altering it around such that you are looking back. I see that I was out of phase a little, so I wrote it myself:

Dim Result As Long, PreResult As Long, oldresult As Long
        Dim y As Long
 
        Me.lb1.Items.Clear()
        Me.lbAll.Items.Clear()
        Result = 2
        PreResult = 1
        y = 3
        Do
            oldresult = PreResult
            PreResult = Result
            Result = Result + oldresult
            Me.lbAll.Items.Add(Result)
            If Result Mod 2 = 0 Then
                y += Result
                Me.lb1.Items.Add(Result)
            End If
        Loop While Result <= 1000000
 
        lMess.Text = y.ToString

I put two listboxes on the form, one shows the even results, the other shows ALL the results. The sequence is correct.

The value of y is: 1,089,155
The highest result is: 832,040